1. Field of the Invention
The present invention relates to a shift control apparatus of an automatic transmission in a vehicle having a constant speed traveling apparatus and an automatic transmission capable of steplessly changing a transmission ratio.
2. Description of the Related Art
Among vehicles, there is one including a constant speed traveling apparatus, which has a detecting device detecting a surrounding situation of the vehicle and operates according to the detected situation, and an automatic transmission capable of steplessly changing a transmission ratio, and having a shift control apparatus controlling the automatic transmission according to a shift map for changing the transmission ratio according to a traveling state. A constant speed traveling control device of the constant speed traveling apparatus performs constant speed traveling control to travel at a preset speed when there is no preceding vehicle, or to maintain a preset inter-vehicle distance when there is a preceding vehicle, according to the surrounding situation of the vehicle detected by the detecting device.
Among conventional shift control apparatuses of an automatic transmission, as disclosed in Japanese Laid-open Patent Publication No. H2-85559, there is one that changes a change width of a transmission ratio of an automatic transmission (continuously variable transmission) in a vehicle having a constant speed traveling apparatus (constant speed traveling control device).
This shift control apparatus of the automatic transmission performs control to suppress the change width during constant speed traveling. Specifically, the apparatus sets an input shaft rotation speed of the automatic transmission to an intermediate band, and limits the control range, which is effective for keeping the traveling speed to be constantly in the vicinity of a set speed of constant speed traveling, and not to deviate largely from the set speed.
Further, among conventional shift control apparatuses of an automatic transmission, as disclosed in Japanese Patent Publication No. 3322207, there is a shift control apparatus having both an active cruise control apparatus as one among constant speed traveling apparatuses (constant speed traveling control devices) that includes a device detecting a surrounding situation of the vehicle, such as a radar sensor, and operates according to the detected situation, and a continuously variable transmission as one among automatic transmission apparatuses.
This shift control apparatus of the automatic transmission varies the gain of acceleration or deceleration between a normal time and a time when the constant speed traveling apparatus is operating.
In the conventional shift control apparatus of the automatic transmission, shift control in constant speed traveling control by the constant speed traveling apparatus assists the constant speed traveling control by the constant speed traveling control device according to a normal shift map as a base. However, one shift map is unable to respond to various traveling situations. For example, the one shift map provides a poor drivability such that a brake hunting occurs due to following only with a brake during traveling on a downhill road, or acceleration does not meet a demand when a preceding vehicle accelerates or during traveling on an uphill road, worsening a following ability.
The technique of Japanese Laid-open Patent Publication No. H2-85559 can give comfort by suppressing speed changes in the case of a vehicle traveling alone with almost no traffic around, or the like, but is still unable to respond to all of a variety of traveling situations of a vehicle.
For example, first, when the constant speed traveling apparatus has an obstacle detecting device such as a radar sensor and adjusts the traveling speed automatically to maintain an inter-vehicle distance, the apparatus needs to perform appropriate control corresponding to the traveling speed of a preceding vehicle or to merging or departing of a vehicle when such an event happens.
Further, control must be performed to give a large acceleration or deceleration in response to a sudden change in the traveling situation, and meanwhile, in a situation with a small change, such a large acceleration or deceleration should rather be avoided to give priority to the comfort.
Also, when there is a change in inclination or declination on an uphill road or down hill road in the road environment, control must be performed to respond also to such change.
In a complex traveling situation of a vehicle including these points, for example, the case where the vehicle becomes slower than a preceding vehicle when returning to a set speed from a speed lower than the set speed on an uphill road, the case where speed control only with shift control and output control becomes insufficient when the vehicle is following a slow preceding vehicle on a steep downhill road, and brake control intervenes intermittently and frequently, or the like, it is most desirable to perform appropriate control to meet a demand responding to expectations in such a traveling situation. Inability to do this gives the driver a feeling of poor following ability or low drivability.
In the system of Japanese Patent Publication No. 3322207, an engine braking control gain (amount of change in transmission ratio per control cycle for engine brake control) is varied between a normal time and a time when the constant speed traveling apparatus is operating, but the acceleration and the deceleration are the same. Accordingly, there is still room for improvement in making a change according to the complex traveling situation of a vehicle when acceleration or deceleration is performed responding to a demand of the constant speed traveling apparatus in operation.